The demand for rechargeable batteries having ever greater energy density has resulted in substantial research and development activity in rechargeable lithium batteries. The use of lithium is associated with high energy density, high battery voltage, long shelf life, but also with safety problems (ie. fires). As a result of these safety problems, many rechargeable lithium battery electrochemistries and/or sizes are unsuitable for use by the public. In general, batteries with electrochemistries employing pure lithium metal or lithium alloy anodes are only available to the public in very small sizes (eg. coin cell size) or are primary types (eg. non-rechargeable). However, larger rechargeable batteries having such electrochemistries can serve for military or certain remote power applications where safety concerns are of somewhat lesser importance.
Recently, a type of rechargeable lithium battery known as lithium-ion or `rocking chair` has become available commercially and represents a preferred rechargeable power source for many consumer electronics applications. These batteries have the greatest energy density (Wh/L) of presently available conventional rechargeable systems (ie. NiCd; NiM, or lead acid batteries). Additionally, the operating voltage of lithium ion batteries is often sufficiently high such that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. 3.6 V lithium ion batteries based on LiCoO.sub.2 /pre-graphitic carbon electrochemistry are now commercially available. Many other lithium transition metal oxide compounds are suitable for use as cathode material, including LiNiO.sub.2 and LiMn.sub.2 O.sub.4. Also, a wide range of carbonaceous compounds is suitable for use as the anode material, including coke and pure graphite. The aforementioned products employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6, salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
Lithium ion batteries can be sensitive to certain types of abuse, particularly overcharge abuse wherein the normal operating voltage is exceeded during recharge. During overcharge, excessive lithium is extracted from the cathode with a corresponding excessive insertion or even plating of lithium at the anode. This can make both electrodes less stable thermally. Overcharging also results in heating of the battery since much of the input energy is dissipated rather than stored. The decrease in thermal stability combined with battery heating can lead to thermal runaway and fire on overcharge. Many manufacturers have decided to incorporate additional safety devices as a greater level of protection against overcharge abuse. For instance, as described in U.S. Pat. No. 4,943,497 and Canadian Patent Application Serial No. 2,099,657, filed Jun. 25, 1993, respectively, the present products of Sony Corp. and Moli Energy (1990) Limited incorporate internal disconnect devices which activate when the internal pressure of the battery exceeds a predetermined value during overcharge abuse.
These pressure activated disconnect devices thus rely on battery constructions wherein the internal pressure is maintained below the predetermined value over a wide range of normal operating conditions yet, during overcharge, the internal pressure reliably exceeds said value. In Canadian Patent Application Serial No. 2,093,763, filed Apr. 8, 1993, a net increase in internal solids volume is employed to hydraulically activate a disconnect device reliably at a specified state of overcharge.
In the aforementioned U.S. Pat. No. 4,943,497, enabling constructions comprise various cathode compounds and/or additives (eg. LiNiO.sub.2, LiNi.sub.y Co.sub.1-y O.sub.2) that result in sufficient gas generation above a certain voltage during overcharge so as to activate the disconnect device. Alternately, in European Patent Application No. 536425, Sony discloses the use of a small percentage of Li.sub.2 CO.sub.3 as a cathode additive that serves as a gassing agent in a similar manner.
Some aromatic compounds containing methyl groups have been used in electrolyte solvent mixtures and/or as electrolyte solvent additives in certain specific rechargeable non-aqueous lithium batteries. For instance, in Japanese Patent Application Laid-open No. 04-249870, toluene is used as an electrolyte solvent and/or electrolyte additive to enhance cycle life. In Japanese Patent Application Laid-open No. 04-332479, toluene, xylene, and mesitylene are suggested for use as electrolyte additives to stop further heat generation from occurring after an internal disconnect device is activated on overcharge. Thus, these additives are not employed for purposes of generating gas per se. Therein, however, it is speculated that methane is produced as a result of oxidation of the toluene at the voltages experienced during overcharge.
Additionally, some aromatic heterocyclic compounds have been used as electrolyte solvent additives for purposes of enhancing cycle life in certain specific rechargeable non-aqueous lithium batteries. In Japanese Patent Applicacion Laid-open No. 61-230276, a laboratory test cell employing an electrolyte comprising a furan solvent additive demonstrated an improved cycling efficiency for plated lithium metal. In Japanese Patent Application Laid-open No. 61-147475, a polyacetylene anode, TiS.sub.2 cathode battery employing an electrolyte comprising a thiophene solvent additive showed better cycling characteristics than similar batteries without the additive.
In European Patent Application No. 614,239, Tadiran disclose a method for protecting non-aqueous rechargeable lithium batteries against both overcharge and overtemperature abuse via use of a polymerizing electrolyte. The liquid electrolyte polymerizes at battery voltages greater than the maximum operating voltage or maximum operating temperature of the battery thereby increasing the internal resistance of the battery and protecting the battery. The method is suitable for lithium batteries employing pure lithium metal, lithium alloy, and/or lithium insertion compound anodes.
It is known in the art that certain aromatic compounds, including heterocyclic compounds, can be polymerized electrochemically (eg. R. J. Waltman et al. investigated the properties of electropolymerized polythiophene in J. Electrochem. Soc., 131(6), 1452-6, 1984.)
Co-pending Canadian Patent Application Serial No. 2,156,800, filed Aug. 23, 1995 by a common inventor, discloses the use of polymerizable aromatic monomers additives for purposes of protecting a rechargeable lithium battery during overcharge. Therein, a small amount of polymerizable aromatic additive is mixed in the liquid electrolyte. During overcharge abuse, the aromatic additive polymerizes at voltages greater than the maximum operating voltage of the battery thereby increasing its internal resistance sufficiently for protection. No mention is made therein about the possible use of similar additives as gassing agents in batteries comprising internal disconnect devices.